Physical processes associated with summer rains in the western slope of the Peruvian Andes, using weather radar data and numerical modeling: case studies

Abstract Rain in the city of Lima is unusual and difficult to predict, so it is of great interest to study the mechanisms that generate it when it exceeds normal limits. This work analyzes the synoptic and local circulation conditions that gave rise two precipitation events over the Rimac river basin, in order to characterize the physical processes related to those events. In the first case, the rain affected the city of Lima. In the investigation, surface precipitation measurements, weather radar and satellite information, as well as the WRF outputs were used. For the synoptic analysis, ERA5 reanalysis database were used. As a result, the role played by the humid Eastern Amazon flow was confirmed, but in this case, the important role played by the local circulation of sea daytime breezes and its interaction with Amazon flow, which determined the zone of formation of the largest precipitation areas, was detected. Associated with this interaction, the presence of gravity waves and their importance in strengthening cloud systems was observed. At the same time, it was detected that the daytime sea breeze does not change direction during the night, as it generally does, but it stays from the sea towards the land, although somewhat weaker. The weakening of the Eastern flow from the Amazon was observed to be related to the retreat to the east of the ridge of the South Atlantic Anticyclone. Also, the importance of anticyclonic circulation at high levels over the region was confirmed. At the same time, it was found that the WRF model acceptably describes the mechanisms of formation of this type of event and also adequately reflects the spatial distribution of the rains associated with them. In general, it underestimated the accumulated precipitation recorded by the rain gauge network in the region.